The present invention relates to a multipolar-magnetized cylindrical permanent magnet to be used as a rotor of a permanent magnet motor or a synchronous motor, such as servomotors and spindle motors, and further relates to a permanent magnet motor including the rotor. More particularly, the invention relates to a multipolar-magnetized cylindrical permanent magnet having magnetic anisotropy in a single diametrical direction, or in a single direction perpendicular to the axis of the cylindrical magnet, as well as to a permanent magnet motor including the magnet as the rotor.
As is well known, permanent magnets having magnetic anisotropy, i.e. permanent magnets capable of being more easily magnetized in a specific direction than in other directions, are widely employed as a part of loudspeakers, electric motors, metering instruments and other electric apparatuses. Such an anisotropic permanent magnet is prepared from a permanent magnet material having crystalline magnetic anisotropy, such as certain hard ferrites and rare earth element-containing alloys. The material is pulverized into a powder of fine particles, followed by compression molding of the powder within a magnetic field (referred to as xe2x80x9cin-field moldingxe2x80x9d hereinafter) to provide a powder compact which is followed by sintering of the powder compact. In the in-field molding of the magnetic powder, the magnetic particles are each oriented relative to the easy magnetization axis of the magnet crystallites as a consequence of the magnetic field applied, so that the resultant sintered magnet also has magnetic anisotropy in the direction of the magnetic field applied to the powder under compression during the in-field molding.
The direction of the magnetic field in the in-field molding of the magnetically anisotropic magnetic particles can be either perpendicular or parallel to the direction of compression for the molding. For example, the anisotropic direction, i.e. the most easily magnetizable direction, of a cylindrical permanent magnet prepared from a powder of a rare earth-based magnetic alloy can be either in parallel to the axial direction of the cylindrical form or in a radial direction perpendicular to the axial direction. Cylindrical rare earth permanent magnets having a radial anisotropic direction are employed as rotors in various types of permanent magnet motors such as AC servomotors, DC brushless motors and the like because of the advantages in that they can be freely magnetized in the axial direction, and no reinforcement is required for assembling unit magnets, as is required in the assemblage of segment magnets. In recent years, a radially anisotropic cylindrical permanent magnet having an increased height or dimension in its axial direction has been desired to meet the needs associated with the expansion of the application fields of permanent magnet motors.
A cylindrical permanent magnet having radial anisotropy is prepared usually by the method of in-field molding, or by the method of backward extrusion molding of the magnet powder. In the in-field molding method, while the magnetic alloy powder in a metal mold is compressed in the axial direction of the cylinder, a magnetic field is applied to the powder under compression in a radial direction through cores at each of the opposite ends of the cylinder. Accordingly, the height, i.e. the dimension in the axial direction of the cylinder, of a radially anisotropic cylindrical magnet is limited by the dimensions or shape of the cores, so that a radially anisotropic cylindrical magnet of an increased height can be prepared only with great difficulties. This method is also not productive because only one molded body can be obtained in a single molding operation using a single molding press. The method of backward extrusion molding is also disadvantageous due to the high cost for the preparation of molded bodies, in that the method requires a large and complicated, and hence very expensive, molding machine. Also, the yield of acceptable molded bodies is relatively low. This situation naturally increases the cost of permanent magnet motors using an expensive multi-radially anisotropic cylindrical permanent magnet as the rotor.
Even without using a multi-radially anisotropic cylindrical permanent magnet, a high-performance cylindrical magnet to be used as a rotor in a permanent magnet motor could be obtained when multipolar magnetization of a cylindrical permanent magnet is accomplished with a sufficiently high magnetic flux density on its surface, and with little variation of the magnetic flux densities among the magnetic poles. In this regard, a method is proposed in the papers of Electricity Society, Magnetics Group MAG-85-120 (1985), according to which a cylindrical magnet having magnetic orientation in a single direction perpendicular to the cylinder axis is prepared by using an in-field molding press under application of a magnetic field in the direction perpendicular to the direction of compression (referred to as a diametrically oriented cylindrical permanent magnet hereinafter). The magnet is provided with multipolar magnetization so that a multipolar cylindrical permanent magnet to serve as a rotor in a permanent magnet motor can be obtained without using an expensive multiradially anisotropic magnet.
The above mentioned cylindrical permanent magnet, which is magnetically oriented in a single direction perpendicular to the cylinder axis (referred to as a diametrically oriented cylindrical magnet), may have an increased height of 50 mm or even larger, if permitted by the dimensions of the cavity of the metal mold, and if a multi-stage molding method can be undertaken. Thus, a plurality of diametrically oriented cylindrical magnets can be obtained by a single operation of compression molding using a multi-cavity metal mold at low costs. Such a diametrically oriented multipolar cylindrical permanent magnet can be employed in place of expensive multi-radially anisotropic magnets as a rotor in permanent magnet motors.
Though possible in principle, the above mentioned diametrically oriented cylindrical permanent magnets are practically infeasible as rotors of permanent magnet motors due to the irregular distribution of magnetic flux density around the circumferential surface of the cylindrical permanent magnet. That is, the magnetic flux density is high on the magnetic pole at or in the vicinity of the direction of the diametrical orientation axis, and is low on the magnetic pole at or in the vicinity of the direction perpendicular to the diametrical orientation axis. This results in the rotational torque of the motor constructed with the rotor being necessarily uneven around the rotation axis, corresponding to uneven or irregular distribution of the magnetic flux density.
The present invention accordingly has a primary object to provide a permanent magnet motor using a diametrically oriented cylindrical permanent magnet as the rotor, without the above described problems and disadvantages, in the conventional permanent magnet motors of similar types. The unexpected discovery leading to the present invention in this regard is that a high-performance permanent magnet motor using a diametricallyl oriented cylindrical permanent magnet as the rotor can be obtained when the number of the magnetic poles of the multipolar-magnetized cylindrical permanent magnet or rotor, and the number of the stator teeth of the stator satisfy a certain specific relationship.
A secondary object of the invention is to provide a novel and improved diametrically oriented cylindrical permanent magnet or rotor having an increased height in its axial direction.
Thus, the permanent magnet motor provided by the invention to accomplish the above described primary object of the present invention is an assembly which comprises:
(a) a stator having a plurality of stator teeth; and
(b) a rotor coaxially inserted into the stator, which rotor is a monolithic cylindrical permanent magnet having magnetic anisotropy in a single diametrical direction perpendicular to the cylinder axis, and being magnetized to have a plurality of evenly disposed magnetic poles around the circumference of the cylinder. The number of the magnetic poles k of the rotor is an even integer not exceeding 100, and the number of the stator teeth n is equal to 3n0, with n0 being a positive integer not exceeding 33, with the proviso that k is not equal to n.
In a particular embodiment of the above defined permanent magnet motor, the diametrically oriented cylindrical permanent magnet to be used as the rotor is provided with multipolar skew magnetization, in which the skew angle of the multipolar skewed magnetic poles is in the range from one tenth to two thirds of 360xc2x0/k.
In a further particular embodiment of the permanent magnet motor, the stator has a plurality of skewed stator teeth, in which the skew angle of the skewed stator teeth is in the range from one tenth to two thirds of 360xc2x0/k.
The above defined relationship between the number of the multipolar magnetic poles k of the rotor and the number of the stator teeth n of the stator can be defined in a different way such that the number of the magnetic poles k is an even number not smaller than 4, and the number of the stator teeth n is equal to 3kxc2x7n0/2, with n0 being a positive integer.
The present invention further provides, in order to accomplish the secondary object of the invention, a rotor in a permanent magnet motor in the form of a composite cylindrical permanent magnet block. The cylindrical permanent magnetic block comprises at least one assembly of at least two or preferably, two to ten cylindrical unit permanent magnets coaxially stacked one on the other, with each having magnetic anisotropy in a single diametrical direction perpendicular to the cylinder axis. The composite cylindrical permanent magnet block is provided with multipolar magnetization to provide a plurality of magnetic poles around the circumference of the cylindrical block.
In a particular embodiment of the above defined rotor for a permanent magnet motor, the direction of diametrical orientation of a first cylindrical unit permanent magnet makes a rotational displacement angle, within a plane perpendicular to the cylinder axis, relative to the direction of diametrical orientation of a second cylindrical unit permanent magnet that is adjacent to the first cylindrical unit permanent magnet. The angle is equal to 180xc2x0 divided by the number of the cylindrical unit permanent magnets stacked together one on the other, assuming that the cylindrical unit permanent magnets each have an identical height with respect to one another.
In a further particular embodiment of the above defined rotor, the number of the magnetic poles around the circumference of the composite cylindrical permanent magnet block is an even number not exceeding 50, and the number of the cylindrical unit permanent magnets coaxially stacked together one on the other is equal to one half of the number of the magnetic poles.
In yet another particular embodiment of the above defined rotor, the composite cylindrical permanent magnet block is magnetized to have a plurality of skewed magnetic poles around its circumference, in which the skew angle of the skewed magnetic poles is in the range from one tenth to two thirds of 360xc2x0 divided by the number of the skewed magnetic poles.